Update README.md about docker images

[dpxcc]

No description provided.

[dpxcc]

@JelteF I ended up updating a few more places that I think are outdated, let me know if you want to keep any of them
Also, we haven't published a 18-main image to be used yet

[JelteF]

The build for 18 is broken. I think it probably never worked and that's why 18-main is unavailable. Can you test building locally and create a PR to fix it: https://github.com/duckdb/pg_duckdb/actions/runs/18737871072/job/53448152201

[dpxcc]

It seems that all docker builds failed after DuckDB upgrade...
Here is the failure for PG 17: https://github.com/duckdb/pg_duckdb/actions/runs/18550616409/job/52877259502

#27 85.80 -- Found OpenSSL: /usr/lib/x86_64-linux-gnu/libcrypto.so (found version "3.0.17")  
#27 85.83 CMake Error at /usr/share/cmake-3.25/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
#27 85.83   Could NOT find CURL (missing: CURL_LIBRARY CURL_INCLUDE_DIR)
#27 85.83 Call Stack (most recent call first):
#27 85.83   /usr/share/cmake-3.25/Modules/FindPackageHandleStandardArgs.cmake:600 (_FPHSA_FAILURE_MESSAGE)
#27 85.83   /usr/share/cmake-3.25/Modules/FindCURL.cmake:182 (find_package_handle_standard_args)
#27 85.83   build/release/_deps/httpfs_extension_fc-src/CMakeLists.txt:18 (find_package)

Will take a look

[dpxcc]

@JelteF
Docker build fix is at #961
We will also need #955 to update docs/compilation.md

[dpxcc]

docker build passed: https://github.com/duckdb/pg_duckdb/actions/runs/18798374304